To effectively work in our visual environment, we use a spatially- selective attention system to focus attention on stimuli of interest and to ignore surrounding stimuli. The overall objective of this study is to understand better the cortical control of shifts and maintenance of visual attention within extrapersonal space. We will study rhesus monkeys and human patients with cerebral cortical lesions. Maintenance and shifts of attention will be determined by measuring the reaction time to push a button in response to a visual target when attention is engaged at the location of the forthcoming target (maintenance of attention) or at a different location (shifts of attention). We will examine attention deficits for targets located at different spatial sites in monkeys and in patients with unilateral posterior parietal (PP) lesions. This will allow us to determine whether spatial attention in PP cortex in each hemisphere is based on a directional organization for shifts of attention, or on a representational organization where contralateral space is more strongly represented in each hemisphere. If a representational organization is found we will determine whether it is based on a craniotopic (head-centered) or somatotopic (body-centered) coordinate system. Furthermore, by changing the spatial location of the button pushing hand with respect to the head and body, we will determine to what extent PP lesions result in a spatial intentional deficit (preparation to move in response to the stimulus). By examining monkeys with unilateral striate cortical lesions + corpus callosum sections, we hope to determine whether striate cortex or subcortical structures are the major source of visual input into cortex in one hemisphere for use in spatial visual attention. By examining deficits in spatial visual attention in individuals with bilateral PP lesions, we hope to determine the true role of this area in spatial attention without any bias effects of a unilateral lesion. Our results will provide a better understanding of the functional and topographic organization of spatial visual attention mediated by cortex.